I figured the headphone section would be a good place to drum up interest in a headphone amp...

Since there are only two tubes they both play into this equation and fairly equally. The big one is the final "amp", the little one is the pre. I've noted differences in bouncing both around. I really like the heavy graphite 6080 tubes - Bendix 6080 Wb's I think they are. Not cheap though. Otherwise I really don't have a preference

In the 12AU7 slot - whatever 12AU7 floats your boat and any of the 12AU7 derivatives as well. What I found was that I really like the sound of the 6SN7 tube - you can get adapters for those on the bay that fit into the 12AU7 socket. So once you've got through the 10,000 flavors of 12AU7's you can visit the 1000 flavors of the 6SN7.